Atomic Absorption Spectra (WJEC GCSE Physics)

Continuous Spectra

• White light contains all the wavelengths and frequencies of the electromagnetic spectrum
• When white light is split up by a prism, all the colours in the visible range can be observed
• This is called a continuous spectrum

Continuous Spectrum of Light

A continuous spectrum is one which shows all wavelengths of light

Absorption Spectra

• When white light passes through a gas, certain wavelengths of light are found to be missing from the spectra
• This is because atoms in a gas absorb light at specific wavelengths
• The specific wavelengths depend on the elements present in the gas
• This produces a continuous spectrum with dark lines at the wavelengths where light is absorbed
• This is called an absorption spectrum

Absorption Spectrum of Light

An absorption spectrum is one which shows all wavelengths of light with dark lines where specific wavelengths have been absorbed

• Every element produces a unique pattern of spectral lines
• Therefore, absorption spectra can be used to identify which elements are present in a gas

Chemical Composition of Stars

• When astronomers study the spectra of stars, they observe the same dark lines as seen in the absorption spectra of known elements on Earth
• This is because the white light produced by a star passes through gases in its outer layers
• As it does so, specific wavelengths of the white light are absorbed

Determining the Chemical Composition of the Sun

The dark lines on the absorption spectrum of the Sun tell us about its chemical composition, which is that it contains hydrogen, helium and sodium, but not calcium or mercury

• By comparing the absorption spectra of a star to the spectra of known elements here on Earth, astronomers can determine the chemical composition of the star
• For example, the absorption spectrum of the Sun tells us that it is predominantly made of hydrogen and helium gas